The use of various types of sensors to evaluate one or more physiological characteristics or parameters of a patient is well known. For example, optical pulse oximetry sensors measure the level of oxygen saturation (SpO2) in a patient's blood. Typically, a light emitting diode (LED) transmits optical radiation of several different wavelengths, e.g., visible and infrared, through blood and tissue of a predetermined portion of a patient's body, such as the wrist or finger. A photodetector detects the light after it passes through the body. Different wavelengths of light are absorbed differently based on blood oxygen content, so detecting the optical attenuation at each wavelength permits the determination of oxygen saturation. In another example, electrocardiogram (ECG or EKG) electrodes are generally planar electrodes connected via wires to an ECG unit that measures the voltage across different pairs of the electrodes to monitor the patient's heart. It is generally required that sensors be correctly placed with respect to a specific body part to be measured. For example, an optical pulse oximetry sensor should be placed so that the optical path from the transmitter to the detector intersects a blood vessel. In like fashion, an ECG sensor should be placed on a part of the body that provides effective electrical contact across the skin (e.g., not on top of significant amounts of hair).
Sensors are constructed in different forms to enable attachment to different portions of a patient's body. For example, optical oximetry sensors can operate by detecting light transmitted through the tissue or light reflected by the tissue. Transmission-mode sensors are useful for fingers and other narrow parts of the body. Reflection-mode sensors are useful for thicker parts of the body, e.g., the forehead or torso. Moreover, sensors are generally calibrated relative to their intended usage. For example, optical sensors are designed and calibrated depending on whether their intended use is as a transmission or reflectance sensor, and will be calibrated for a specific spacing, or range of spacings, between the emitters and the detector. Thus, even two transmission sensors, such as one intended for use on a fingertip and another intended for use on an earlobe, will typically have different calibrations. The calibration differences between a transmission sensor and a reflectance sensor are typically greater.
Sensors are designed for specific locations on the body, as discussed above. Given this specificity, it can be difficult for caregivers to apply sensors correctly. This situation is exacerbated when patients must properly apply sensors to themselves, e.g., in outpatient or home-care situations. For example, a bandage-type transmission sensor intended for use on a fingertip, and which would normally be folded over or around the fingertip, may be unfolded and applied to another portion of the patient's body in a configuration like a reflectance sensor. However, in such a circumstance, not only are the placement of the sensor and measurement method different from what was intended, but the spacing between the emitters and detector is also significantly different from what was intended for the sensor. Thus, the misapplied sensor will not give accurate readings for the patient. Other misapplications of a sensor include placement on a site which, although positionally correct, is not suitable for optimal measurements. This situation may exist, for example, when the physical characteristics of the site are unsatisfactory to yield reliable measurements, e.g., due to sweat, hair, or position of subcutaneous fat. For example, although an oximeter calibrated for the pointer finger may be intended for use with either finger, differences between the patient's two pointer fingers may only permit the oximeter to be effectively used with one of those fingers. Moreover, the user or health care provider may unwittingly or carelessly position a physiologic sensor in whole or in part over an article of clothing.